x86: Consolidate PCI_MMCONFIG configs

Since e279b6c1

 ("x86: start unification of arch/x86/Kconfig.*"), there
exist two PCI_MMCONFIG entries, one from the original i386 and another from
x86_64. Consolidate both entries into a single one.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: jailhouse-dev@googlegroups.com
Cc: linux-pci@vger.kernel.org
Cc: virtualization@lists.linux-foundation.org
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lkml.kernel.org/r/2a0ccd51ea6f7996e07162918228e23bdc1fbb03.1520408357.git.jan.kiszka@siemens.com